The present invention relates to a multiple output power supply for providing stable multiple output voltages, and especially to a multiple output power supply with a constant volt-second clamp circuit for providing stable multiple output voltages.
FIG. 1 shows a multiple output power supply according to the prior art. Please refer to FIG. 1. A typical multiple output power supply includes a voltage supply 1, a transformer 2, a feedback controller 3, and a post regulator 4. The voltage supply 1 provides a supply voltage for the multiple output power supply. The transformer 2 includes a primary winding 21 and at least two secondary windings 22 for providing multiple output voltages. The post regulator 4 is electrically connected to the secondary windings 22 for providing stable output voltages. The feedback controller 3 is electrically connected to the secondary windings 22 for feeding back output voltages in order to control a duty cycle of an input voltage at the primary winding 21 of the transformer 2. If the output voltages increases, the duty cycle will decrease to stabilize the output voltages. The feedback controller 3 will adjust the duty cycle of the input voltage at the primary winding 21 of the transformer 2 if there are variations at any one of the output voltages. However, it is not necessary for the other output voltages to adjust the duty cycle of the input voltage at the primary winding 21 of the transformer 2 for compensating the variations at one of the output voltages if there are variations at one of the output voltages. Thereby, the over-compensating effect will occur where the voltage across the post regulators 4 and the power dissipation at the post regulators 4 are increased. The over-voltage across the post regulators 4 increases the possibility of damage to the post regulators 4.
It is therefore attempted by the applicant to deal with the above situation encountered by the prior art.
It is therefore an object of the present invention to propose a multiple output power supply with a constant volt-second clamp circuit for maintaining a constant volt-second product at the input voltage, which is independent of the load effect of output voltages and variations in input voltages.
According to an aspect of the present invention, a multiple output power supply for providing stable multiple output voltages includes a transformer having a primary winding electrically connected to a supply voltage terminal and at least two secondary windings for providing multiple output voltages, and a constant volt-second clamp circuit electrically connected to the primary winding for achieving a constant volt-second product clamp over a varying input voltage at the primary winding, in which the constant volt-second product clamp over the varying input voltage at the primary winding is independent of a load effect of the secondary windings.
Preferably, the secondary winding is electrically connected to a post regulator.
Preferably, the post regulator is a magnetic amplifier regulator.
Preferably, the post regulator is a synchronization rectifier.
Preferably, the post regulator is a linear regulator.
Preferably, the constant volt-second clamp circuit includes a sawtooth wave generator for providing a sawtooth wave that increases linearly as a function of time with a slope which is proportional to a supply voltage, a clock generator for providing a timing signal, a first transistor switch having a control electrode electrically connected to the clock generator, a first conduction electrode electrically connected to ground, and a second conduction electrode electrically connected to the sawtooth wave generator for being turned on when the timing signal is at a low voltage level and turned off when the timing signal is at a high voltage level so that the sawtooth wave generator outputs the sawtooth wave when the timing signal is at the low voltage level, a comparator having a noninverting terminal electrically connected to a first voltage, and an inverting terminal electrically connected to an output of the sawtooth wave generator, a first NOR gate having a terminal electrically connected to an output of the clock generator, and the other terminal electrically connected to an output of the comparator, a second NOR gate having a terminal electrically connected to the output of the clock generator, and the other terminal electrically connected to an output of the first NOR gate, and a second transistor switch having a control electrode electrically connected to an output of the second NOR gate, a first conduction electrode electrically connected to one end of the primary winding of the transformer, and a second conduction electrode electrically connected to one end of the supply voltage terminal.
Preferably, the first transistor switch is a bipolar junction transistor (BJT).
Preferably, the second transistor switch is a metal oxide semiconductor field effect transistor (MOSFET).
Preferably, the sawtooth wave generator includes a resistor having one end electrically connected to said supply voltage, and a capacitor having one end electrically connected to the other end of the resistor to form a node that is the output of the sawtooth wave generator, and the other end electrically connected to ground.
Preferably, the constant volt-second clamp circuit includes a sawtooth wave generator for providing a sawtooth wave that increases linearly as a function of time with a slope which is proportional to the supply voltage, a pulse width modulation (PWM) generator for providing a pulse width modulation signal, a diode having a cathode end electrically connected to an output of the pulse width modulation generator, and an anode electrically connected to an output of the sawtooth wave generator for being turned on when the pulse width modulation signal is at a low level voltage and turned off when the pulse width modulation signal is at a high level voltage so that the sawtooth wave generator outputs the sawtooth wave when the pulse width modulation signal is at the high level voltage, a comparator having a noninverting terminal electrically connected to a first voltage level, and an inverting terminal electrically connected to an output of the sawtooth wave generator, a first transistor switch having a control electrode electrically connected to an output of the comparator, a first conduction electrode electrically connected to an output of the pulse width modulation generator, and a second conduction electrode electrically connected to ground, a push-pull circuit having an input terminal electrically connected to the first conduction electrode of the first transistor switch, and a second transistor switch having a control electrode electrically connected to an output of the push-pull circuit, a first conduction electrode electrically connected to one end of the supply voltage terminal, and a second conduction electrode electrically connected to one end of the primary winding.
Alternatively, the constant volt-second clamp circuit includes a pulse width modulation (PWM) generator for providing a pulse width modulation signal, a peak holding circuit electrically connected to one end of said secondary windings for obtaining a voltage, a sawtooth wave generator for providing a sawtooth wave that increases linearly as a function of time with a slope which is proportional to the supply voltage, a diode having a diode having an anode end electrically connected to an output of the sawtooth wave generator, and a cathode end electrically connected to an output of the pulse width modulation generator for being turned on when the pulse width modulation signal is at a low level voltage and turned off when the pulse width modulation signal is at a high level voltage so that the sawtooth wave generator outputs the sawtooth wave when the pulse width modulation signal is at the high level voltage, a comparator having an inverting terminal electrically connected a voltage level, and a noninverting terminal electrically connected to an output of the sawtooth wave generator, a first transistor switch having a control electrode electrically connected to an output of the comparator, a first conduction electrode electrically connected to ground, and a second conduction electrode electrically connected to an output of the pulse width modulation generator, a push-pull circuit having an input terminal electrically connected the second conduction electrode of the first transistor switch, an isolated transformer having a secondary winding electrically connected to an output of the push-pull circuit, and a second transistor switch having a control electrode electrically connected to one end of a primary winding of the isolated transformer, a first conduction electrode electrically connected to the other end of the primary winding of the isolated transformer, and a second conduction electrode electrically connected to one end of the primary winding of the transformer.
Preferably, the peak holding circuit includes a first diode having an anode end electrically connected to one end of the secondary winding of the transformer, and a first capacitor having one end electrically connected to a cathode end of the first diode for forming a node which is an output of the peak holding circuit, and the other end electrically connected to ground.
The present invention may best be understood through the following description with reference to the accompanying drawings, in which: